1. Field of the Invention
The present invention relates broadly to bonding one article to another. More particularly, the invention relates to bonding of leads to substrates, for example, in the manufacture of integrated circuits wherein lead frames are bonded to metallized patterns on ceramic substrates.
2. Description of the Prior Art
In the manufacture of integrated circuits one assembly method includes bonding semiconductor chips to ceramic substrates. Such substrates typically have conductive "sunburst" patterns on one side thereof. The semiconductor chips or devices are usually bonded to centers of the sunburst patterns while a plurality of leads, held in position by what is commonly referred to in the industry as a lead frame, are bonded to outer terminations of these sunburst patterns. After the leads have been bonded to the substrates, the resulting product is usually further processed as, for instance, by molding protective covers over the substrates. Also, at some time after the bonding operation, the leads of the lead frames are cut to form the individual leads. The leads may then also be bent or formed for certain types of integrated circuit packages.
The resulting circuit packages are used in various combinations inserted and soldered into printed circuit boards, where such packages serve as basic electronic building blocks for a variety of electronic systems. In some applications, special package connectors permit the packages to be removably inserted where the packages are not soldered, into the boards. In either case, the insertion of the circuit packages directly into printed circuit boards, or into connectors, subjects the bonded leads to stresses. In addition to such insertion stresses, the leads are subjected to stresses through gradual warping of circuit boards or through thermal expansion and contraction of the leads and the boards or connectors. In the past, it has been a problem to insure that the bonds of the leads to the substrates are consistently of a sufficient strength to withstand such stresses.